Session Initiation Protocol (SIP) is a communications protocol for signaling and controlling multimedia communication sessions, such as Internet telephony applications, multicast sessions, etc. A SIP user agent (UA) is a logical network end-point used to create or receive SIP messages and thereby manage a SIP session. A SIP UA may perform the role of a user agent client (UAC) which sends a SIP request, or a user agent server (UAS), which receives the SIP request and returns a SIP response.